Antenna systems of that type are mainly used for the measurement of filling levels of filling products of any kind, in particular of bulk goods and fluids in storage tanks or receptacles. The determination of the filling level height thereby ensues by measuring the transit time of a microwave pulse, which is emitted by means of the antenna system into the direction of the surface of the filling level to be determined, is reflected from the surface and received by the antenna system, and which is finally transmitted to a combined emitting and receiving module. Through the signal transit time and the wave propagation speed, the path travelled by the signal and, with knowledge of the receptacle height, the filling level may then be determined. For emitting the microwave pulses necessary for this purpose, horn antennas inter alia are used.
Horn antennas are already known from prior art in the field of the filling level measuring technology; however, all of them exhibit certain problems due to their constructional configuration and length.
The microwaves necessary for the level measurement are generated by a high-frequency module (HF module). According to prior art, these microwave signals are laterally coupled into a waveguide via an exciter pin. The exciter pin, which is in most cases used in conjunction with a glass feedthrough, is thereby fed via a coaxial cable from the HF module. From German utility model DE 94 12 243 U1, a horn antenna is known, having an exciter pin connected to a coaxial cable for the level measurement, which exciter pin engages into the lateral wall of a cup-like metal shell. Following this cup-like metal shell is a waveguide. The waveguide in turn is followed by a thereto coupled antenna horn.
A further development of this arrangement consists in providing a planar structure (patch) instead of the exciter pin, so as to couple in turn the microwave pulses into a waveguide, as well. The planar structure is thereby applied on a dielectric material having a dielectric constant of ∈1, and is located on one end of a waveguide. It is proposed in DE 198 00 306 A1, to carry out the coupling of microwaves into a waveguide with the assistance of planar radiation elements. The planar radiation elements are thereby provided on a front end of the waveguide. This solution, however, is again based on the construction-inherent problem that such an antenna system requires a large constructional space—which is mainly extended in the longitudinal direction.
All kinds of feeder systems known, however, have in common that the microwave pulses are coupled into a waveguide, to which follows an antenna horn flaring in the propagation direction. Hereby, the exact matching of the waveguide dimensions to the transmission frequency range of the microwave signal is problematic for allowing only the fundamental mode of wave to propagate within the waveguide. Matching adjustments for the waveguide, which are not exact enough, can lead to signal distortions due to wave dispersion, eventually resulting in a misrepresented measurement result.